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Topic: Unusual outcome to simple reaction I can't explain  (Read 2622 times)

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Offline Sir_Goat

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Unusual outcome to simple reaction I can't explain
« on: March 30, 2015, 08:25:53 AM »
So it's just your basic run-of-the-mill add a base to a compound and attach another substituent to it. Or it should be anyway.

I've used methanol as solvent, added my base, potassium tert-butoxide, and allowed it to equilibriate for 10-15 minutes. Then I added the 2,4-pentanedioneCC(=O)CC(=O)C which was left again for 5 minutes followed by the addition of 4-nitrobenzyl bromideBrCC1=CC=C([N+]([O-])=O)C=C1

The mixture was then left stirring overnight and the solvent then removed via rotary evaporation. The product was analysed with NMR in CDCl3 and it seems to have not attached in the centre but at the end like so CC(CC(CCC1=CC=C([N+]([O-])=O)C=C1)=O)=O

Why has this happened? I added a 1:1 base mixture and surely the hydrogens on the 3- postion should be more acidic?

To make matters worse I'm struggling to recreate my results (basically no reaction occurs on some attempts).

Any suggestions as to why it reacts like it does or why I'm struggling to recreate my results would be much appreciated.  :)
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Offline TheUnassuming

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Re: Unusual outcome to simple reaction I can't explain
« Reply #1 on: March 30, 2015, 09:34:48 AM »
Are you following some manner of lit prep for this or similar reaction?
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Offline Sir_Goat

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Re: Unusual outcome to simple reaction I can't explain
« Reply #2 on: March 30, 2015, 11:46:12 AM »
No, but there are plenty of examples of lit preps with the same basic idea: add a base to the reactants in a solvent.

However, I have just realized that most of them used non-protic solvents as oppose to protic ones like methanol. Maybe this is what is causing me the problem?
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Offline TheUnassuming

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Re: Unusual outcome to simple reaction I can't explain
« Reply #3 on: March 30, 2015, 12:41:33 PM »
I would try to follow what is most common in the literature as closely as possible. 
That said the general reaction scheme you've shown is done in a number of different solvents with many different bases.  So if the method you posted first doesn't work or is finicky, try some other common systems like NaH/THF or carbonate/acetone.
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Offline clarkstill

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Re: Unusual outcome to simple reaction I can't explain
« Reply #4 on: March 30, 2015, 01:20:16 PM »
Just use a weaker base? Tert-butoxide is probably overkill to deprotonate a 1,3-diketone (the pKa of tBuOH in DMSO is over 30!), so you're getting double deprotonation and the more reactive (less stabilized) enolate reacts. I would've thought K2CO3 in something polar and aprotic (THF, MeCN, DMF) would be appropriate.
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Offline orgopete

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Re: Unusual outcome to simple reaction I can't explain
« Reply #5 on: April 02, 2015, 07:33:41 PM »
I'm a bit skeptical. Please post the NMR.
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